System.IO.Ports.SerialPorts is the class which reads serial data. If you are using Winforms then you can simply drag and drop the SerialPort from toolbox. This is how you can use serial port to read data

After doing this you should see the temperature posted on your twitter account, you can modify this program to send direct message instead of tweeting.

Possibilities

I feel this is a basic and a good project for getting started with Arduino and your first step to InternetOfThings. These are various ways I can think off to enhance it, if you have some more geeky ideas, I would love to hear them.